1. Technical Field
The disclosure is related generally to designing test pattern layouts to isolate an effect of interest when competing effects exist. More particularly, the disclosure is related to the design of a test pattern layout for a test photomask, a test photomask and a method for evaluating and isolating critical dimension changes in the test photomask.
2. Related Art
A semiconductor chip is built on a wafer using many layers of material and many imaging steps to form the semiconductor devices. The material layers are often patterned with a specific layout or design that is first created on a photomask before being transferred to the semiconductor wafer. The photomask contains the desired pattern to be printed on a semiconductor wafer for a given layer of the design. It is critical that all errors on the photomask be minimized to enable full functionality of the semiconductor devices. The pattern is created in the photomask translating the designer's intent into clear and opaque regions in an absorbing layer of the photomask. The photomask patterning process is achieved, for example, by applying a photoresist and using an electron beam to expose some regions. A subsequent develop of the resist creates a soft mask for an etch to transfer the resist pattern into the absorber layer. The resist is then stripped and the pattern quality verified on the photomask.
When using pattern generators to write or print patterns on the photomask, it is desirable to have good uniformity of the features of interest. The metric and terminology commonly applied to these features on photomasks is critical dimension (CD) uniformity. Good CD uniformity may substantially ensure that features in the photomask will result in the pattern being transferred accurately when deployed as part of the wafer photolithography step. Effective transfer is important because the photomask design is replicated thousands of times on wafers for the purpose of building semiconductor structures.
During the photomask patterning process, a variety of errors can occur, which may inhibit CD uniformity. Two common errors that occur when using an electron beam pattern generator to write patterns in semiconductors are fogging effects and develop loading effects. These errors and the degree/effect of the errors may depend upon a number of factors, including: electron beam condition, the configuration of the pattern on the photomask, the materials of the photomask, the composition of the photoresist exposed to the electron beam to create a removable mask of the pattern, the “developer” solvent used to remove exposed portions of the semiconductor structure, etc.
To compensate for these errors in photomask patterning, photomask manufacturers often create test photomasks and run test patterning processes to determine how the CD uniformity will be affected by errors. After running and analyzing the test photomask process, a final photomask may be created that may compensate for the anticipated errors that will occur when writing or printing the pattern on the final semiconductor structure. Additionally, to compensate for the detected errors during the test photomask process, manufacturers may adjust or modify pattern generator conditions used in writing or printing the pattern by: adjusting the exposure strength, adjusting the exposure beam conditions, etc.
The use of a test photomask and/or adjustments to the final photomask may not completely eliminate CD non-uniformity within the semiconductor structure if the individual contributors to each error cannot be identified and separated. That is, although the test photolithography process may determine the error effects on CD uniformity on the photomask, it may not be determined how much of an effect fogging has on the CD uniformity, compared to the effects of develop loading effects. As such, the final photomasks are created to compensate for overall effects with the aspirations of substantially minimizing errors and/or maintaining CD uniformity during the photomask patterning process on all features. However, without knowing the exact effect of each error (e.g., fogging, develop loading, etch loading) on the photomask patterns, CD uniformity cannot be appropriately corrected.